Warm Pacific may cause US winter freeze, study finds

Unusually warm western Pacific waters linked to global warming may be the paradoxical cause of a bone-chilling winter in parts of the United States this year, a scientific study said on Thursday.

The theory contrasts with other experts' views, including that the freeze was simply a freak natural event or that it was linked to a thawing of the Arctic in recent years that sent a blast of cold air south.

Eastern US had a series of winter storms earlier this year. Photo: Reuters

"People's reaction when they sit under 10 feet of snow is to say 'this cannot be man-made climate change'," said Professor Tim Palmer of Oxford University, who published his research in the journal Science. "But there is a plausible link," he told Reuters.

He said a strengthening of trade winds had led to a build-up of warm water in the western tropical Pacific, aggravated in recent years by global warming from man-made emissions of greenhouse gases.

Thunderstorms linked to the warmth in turn disrupted the jetstream, high altitude winds which flow in vast meandering loops around the northern hemisphere, and sucked cold air from the Arctic. Detroit, for instance, suffered record snows and the coldest January since 1977.

Pinpointing the causes of the US chill, when climate change should make cold winters less likely, would help companies, farmers, city planners or even home owners wondering if they should invest in extra roof insulation.

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Two other experts were unconvinced by Palmer's study.

Arctic link

Jennifer Francis of Rutgers University, who wrote in 2011 that a melting of Arctic ice may cause cold snaps, said the Pacific had a similar pattern of heavy rainfall in 2011-12 but the winter was mild in the United States.

"In both cases the jet stream's path was extremely amplified or wavy, which is exactly the sort of behaviour we expect to occur more frequently in association with rapid Arctic warming," she told Reuters.

She said that the tropics might also be contributing, but that there seemed little evidence of this.

Martin Hoerling, of the US National Oceanic and Atmospheric Administration's Physical Sciences Division, said he reckoned the most plausible explanation of the cold North American winter was a "freak of nature".

He said that there was no sign of a link between Pacific sea temperatures and US winters in records from 1948 to 2012. And he also said Francis's Arctic theory "has not been affirmed by subsequent studies by a variety of researchers".

So far there is limited understanding of how weather in one part of the world can affect another.

Weather experts agree, however, that the El Nino weather phenomenon that mainly cools the eastern Pacific Ocean every few years can cause droughts or downpours on other continents.

Palmer told Reuters that his theory, building on a 1980s study he wrote suggesting a link between a chill 1976-77 US winter and a warm Pacific, could be tested because there are signs that an El Nino will form later this year.

An El Nino would also cool the western Pacific and that meant a cold US winter was less likely in 2014-15, he said.

A U.N. panel of climate scientists says it is at least 95 per cent probable that human activities, led by burning fossil fuels, are the main cause of warming since the 1950s, and will cause more heatwaves, floods and rising sea levels.